1. Field of the Invention
This invention relates to an apparatus for monitoring the stitching quality of sewing machines and, in particular, to detecting skipped stitches for chainstitch sewing machines.
2. Description of the Related Art
With the clothing industry becoming increasingly automated, there is a need for systems that monitor and regulate the functions and output of high speed sewing equipment. Certain of these systems are utilized to monitor the stitching of sewing machines to detect skipped stitches in apparel manufactured by automated sewing machines.
In the general, improper stitches may from time to time be introduced in a workpiece manufactured with the use of an automated sewing machine. Generally, improper stitches may have the form of malformed stitches or skipped stitches. The incorporated reference U.S. patent application Ser. No. 557,852 describes malformed stitches and skipped stitches that arise in connection with lockstitch (class 301) sewing machines.
In the prior art, particularly for lockstitch sewing machines, skipped stitch detection systems are based upon monitoring the tension of the needle thread. As an example of this system, in U.S. Pat. No. 4,102,283 (Rockerath et al.) the loss of thread tension generally is said to correspond to a skipped stitch, and this reduction in normal thread tension triggers a sensing device. The sensitivity of these systems ranges from complete loss of thread tension, for example due to the thread breaking, to sensing a momentary reduction in normal thread tension. This system would be unable to effectively detect a triangle skip stitch.
Other systems are based upon monitoring thread consumption, and may correlate thread consumption with total number of stitches, to detect a skipped stitch. As an example of this system, in U.S. Pat. No. 3,843,883 (DeVita et al., Oct. 22, 1974) a monitor is used to measure thread consumption which is then compared to a predetermined standard of thread use, deviation from which activates an output signal.
A system used for detecting skipped stitches in a lockstitch type 301 sewing machine is disclosed in UK Patent Application No. GB 2008631. That system involve monitoring the length of a seam as compared with the upper thread consumption required to produce the seam. Actual thread consumption is then compared against a predetermined consumption value, any difference of which corresponds to an improperly formed seam. However, the difference in upper thread consumption between correct stitches and skipped stitches is not always substantial enough to be reliable in fast-rate sewing machines. This is best demonstrated when two pieces of thin fabric are being sewn together. Generally, measurements of the difference in thread consumption per stitch includes the thickness of two plies of fabric (assuming the stitch is set at center). For example, letting stitch length (SL)=0.124 inches, and ply thickness (PT)=0.01 inches, then the percentage decrease for a skipped stitch would be: 100* [(2* PT)/SL]=100 * [(2*0.010)/0.125]=16%. If thread tensions are not adjusted properly, this percent decrease could go to zero. Thus, there is a need for a direct, effective method of detecting skipped stitches in a fast-speed lockstitch type 30 sewing machine.
A primary shortcoming of the prior art is the unreliablity of these systems at high sewing speeds, for example greater than 5,500 stitches per minute. DeVita states that the apparatus disclosed therein makes "mechanically possible the very high running speeds of about 2,000 stitches per minute desirable for such [lockstitch] sewing machines" (emphasis added). These systems fail to detect a momentary reduction of thread tension when the sewing machine is operating at high sewing speeds. The reduction in tension for an improper stitch at high sewing speeds tends to be less and in a range that the prior art fails to detect. As a result, these systems tend to be less reliable and thus fail to perform these functions with great accuracy.
The Class 400 chainstitch is employed in a wide range of areas within the apparel industry because it provides a fast, economical, resilient, and strong stitch chain. The Class 400 stitch tends to be very elastic and is well suited for seaming operations, for example, inseaming pants and closing synthetic bags, on wovens and knits of many types and weights of materials. However, in Class 400 chainstitch, malformed or skipped stitching tend to weaken the entire stitch chain and, as a result when included in the final product, the defective product may prematurely fail, for example for unraveling.
The 400 Class "multi-thread chainstitch" is formed by a sewing machine passing one or more needle thread loops through the material. Those needle thread loops are interlooped on the underside with a looper thread supported on a looper. As an exemplary Class 400 chainstitch, stitch type 401 is formed with two threads, the needle thread and the looper thread. An angularly reciprocal looper, located underneath the material, engages the needle loop projected by an axially reciprocal needle underneath the material. The looper retains the needle loop the looper thread from the previous stitch through the needle loop. The needle then penetrates the material again between the looper thread and the previous needle loop. As a result, when the looper retracts, the needle thread, which comprised the needle loop, tightens and thus completes a stitch. A more detailed description of the chainstitch type 401 is provided in Union "Special Stitch Formation Type 401" brochure, published by Union Special Huntley, Ill. (1979). Collectively, these malformed and skipped stitches are referred to as "improper stitches" hereinbelow. There are many causes of improper stitches. Malformed stitches can develop from improper synchronization between the active elements within the sewing machine and the needle and looper thread loops. In particular, the malformed stitches are formed when the needle thread loop around the blade of the looper is improperly positioned and as a result the needle on its downward travel can enter this loop, forming a "101-type" stitch.
In general, skipped stitches also result from improper synchronization of the needle thread loop and the looper thread loop and may also occur from deflection of the needle. There are primarily two types of skipped stitches: the "needle loop" skip and "triangle" skip. The needle loop skip develops when the looper fails to enter the needle loop and as a result the upward motion pull the loop to the top of the fabric. The triangle skip is formed not by the looper failing to enter the needle loop, but when the needle fails to enter the looper loop. Consequently, since the needle loop was picked up by the looper, the needle thread remains in the material or is loose on the top side of the fabric.
U.S. patent application Ser. No. 332,227 filed on Mar. 31, 1989, entitled "Method and Apparatus For Detecting Improper Stitches For A Chainstitch Sewing Machines," now U.S. Pat. No. 4,991,528, describes a method and apparatus for detecting improper stitches in a chainstitch sewing machine based upon thread consumption over the stitch formation cycle.
However, there continues to exist a need for better methods and systems for detecting skipped stitches for a chainstitch sewing machine that are reliable at high sewing speeds To accommodate the advances in the clothing automation, particularly the increase in sewing speeds, it is an object of the invention to provide a simple, reliable system for detecting skipped stitches that would satisfy a substantial need in the field.